Organization (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2018 USD ($) segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Number of reportable segments | segment									1
Accumulated deficit	$ 369,014								$ 369,014	$ 360,877
Net loss	(5,242)	$ 1,069	$ 4,651	$ 7,659	$ 5,407	$ 2,388	$ 9,778	$ (287)	8,137	17,286	$ 19,566
Cash and cash equivalents	18,286							$ 22,978	18,286	26,862	$ 22,978	$ 14,581
Short term investments	$ 64,638								$ 64,638	$ 0